1. Field of the Invention
The present invention relates to an image reading apparatus such as an image scanner and an image reading lens system mounted on the image reading apparatus.
2. Description of the Related Art
Generally, image reading apparatuses such as copiers, facsimiles, and image scanners for reading an original image such as a document or an image and converting the image into digital image data have come into widespread use. In such image reading apparatuses, by using an imaging lens system (hereinafter referred to as an image reading lens system), the original image is formed on, for example, a CCD image sensor. In order to acquire image data with high fidelity to the original image, in the image reading lens system, various aberrations are suppressed with good balance, and thus a high resolving power is demanded. Furthermore, high aperture efficiency is demanded even in the corner of the screen.
Further, recently, in accordance with reduction in the size and cost of the image reading apparatuses, the image reading lens system has been further demanded to be reduced in size and cost. Further, in accordance with an increase in the image reading speed of the image reading apparatus, a faster (smaller F number) image reading lens system is demanded.
As the compact image reading lens system, there has been known a telephoto type lens system having four groups and four elements (for example, refer to JP-A-9-101452). Further, as the faster image reading lens system, there has been known a Gauss type lens system having four groups and six elements (for example, refer to JP-A-9-113802). Furthermore, as disclosed in JP-A-2000-241701, there has been an image reading lens system capable of achieving both of an increase in speed and reduction in size by adopting a configuration having five groups and five elements.